Recently, water absorbent resins are widely used as a main constituent ingredient of a sanitary material (absorbing article) such as a disposable diaper, a sanitary napkin, and an incontinence pad so as to absorb body fluids (e.g., urine and blood). Examples of known water absorbent resins encompass: a partially neutralized cross-linked polymer of polyacrylic acid; a hydrolyzed starch-acrylonitrile graft polymer; a neutralized starch-acrylic acid graft polymer; a saponified vinyl acetate-acrylic ester copolymer; a cross-linked carboxymethyl cellulose; a hydrolyzed or cross-linked acrylnitryl copolymer or acrylamide copolymer; a cross-linked cationic monomer; a cross-linked isobutylene-maleic copolymer; and a cross-linked polymer of 2-acrylamide-2-methylpropanesulfonate and acrylic acid.
It has conventionally been desirable that the above water absorbent resins have such water absorbing properties as: a high absorption capacity for aqueous liquids including body fluids, particularly a high water absorption capacity under load; a high absorbing rate; an excellent liquid permeability; a high gel strength of a swelling gel thereof; and a high water absorption capacity to absorb water from a substrate in which an aqueous solution is contained.
For achievement of the above water absorbing properties, the above water absorbent resins are each a hydrophilic resin having a uniform cross-linked structure inside the polymer, thereby being insoluble in water. The water absorbent resins normally have particles having a surface cross-linked by, e.g., a cross-linking agent so that the particles have a gradient cross-linking density. This aims to improve each of the water absorbent resins in its water absorption rate, prevention of generation of fish eyes, gel strength, absorption capacity under pressure, prevention of gel blocking, and liquid permeability (see Patent Literatures 1 through 6).
For improvement of its water absorbing performance, for example, a particulate water absorbing agent is known that contains a water absorbent resin having particles having been subjected to a surface cross-linking treatment in the vicinity of the surface so as to have a gradient cross-linking density (Conventional Example 1).
In addition to the above water absorbing agent, a water absorbing agent is known that contains a water absorbent resin and metal soap so as to, e.g., improve its liquid permeability. Also, it is desired that a particulate water absorbing agent be available that not only has high water absorbing performance, but also has no mater whether it is dry or moist, a good powder fluidity and little adhesion to devices etc. during its production and transportation, and during production of an absorbent core from the water absorbent resin and a fiber material. To achieve a good powder fluidity when particulate water absorbing agents have absorbed moisture, addition of inorganic substance powder such as amorphous silicon dioxide or kaolin has been proposed. Known examples of particulate water absorbing agents containing inorganic substance powder encompass: a particulate water absorbing agent including inorganic substance powder and water absorbent resin powder; and a particulate water absorbing agent including stearic acid and inorganic substance powder (Conventional Example 2).
Other known examples encompass: a particulate water absorbing agent containing quaternary ammonium salt; a particulate water absorbing agent containing denatured polysiloxane as well as polyethylene glycol or polypropylene glycol; and a particulate water absorbing agent containing a polymeric dispersant (Conventional Example 3).
Still another known example is a particulate water absorbing agent containing a water absorbent resin having been treated with a surface active agent so as to prevent generation of fish eyes and gel blocking (Conventional Example 4).
Furthermore, a method for improving bulk density has also been proposed. For example, polishing cross-linked polymer particles and then carrying out surface cross-linking with respect to the particles allows for production of irregularly pulverized-shape water absorbent resin powder having a high bulk density and a high absorption capacity under pressure (Conventional Example 5).
Patent Literature 1, for example, discloses a particulate water absorbing agent produced by adding a surface cross-linking agent to a water absorbent resin having a cross-linked structure for surface cross-linking treatment and then adding a surface active agent or a powdered lubricant to the resin. Further, Patent Literature 2 discloses a superabsorbent polymer containing a monomer having a polymerizable, unsaturated acid group, an internal cross-linking agent, a surface cross-linking agent, a permeable modifier, polyhydric metal salt, a surface active agent, and insoluble inorganic powder. Another known water absorbing agent contains a water absorbent resin cross-linked or covered by a surface cross-linking agent in the vicinity of its surface, the water absorbent resin containing polyhydric metal salt and water-insoluble inorganic particles at least in its surface or in the vicinity of the surface.